The invention relates to an apparatus and a process for mixing and applying bone cement. An apparatus of this kind is known, for instance, from EP-A1-170 120.
The components of artificial joints are usually anchored to the bony bed using cold polymerizing two-components resins called bone cement. The bone cement hardens as soon as it has been applied and achieves an anchoring of the prosthesis component into the bony bed by means of its plastic properties. Polymethylmethacrylates (PMMA) have been used for more than 20 years as bone cement; they comprise powdery bead polymers superficially dissolved in a liquid monomer and are finally embedded therein by polymerisation. In the mixing phase the monomer surrounds the pellet-shaped bead polymer. This first leads to a bloating of the pellets in which a considerable amount of air bubbles are trapped. The chemical reaction of the bone cement is initiated by a starter reaction, wherein dibenzoyl peroxide is usually activated by an activator, i.e. p-amino-toluidine, and then a radical chain polymerisation is initiated. This process of polymerisation takes place exothermically. In addition to the trapped air bubbles, when the polymer pellets are surrounded by the monomer, so-called "lee phenomena" appear regularly. Also, the monomer liquid evaporates during the process of exothermic polymerisation which leads to the fact that in the end the hardened polymer is riddled with various bubbles of different ethiology and genesis.
As a rule, the polymer powder or prepolymer is added to the monomer and mixed in a bowl using a spatula. In the processing phase which follows the mixing phase, the bone cement is applied to the bony bed, for instance to the femoral medullary canal or the bony acetabulum, both of which have been prepared for the anchoring of the cemented prosthesis components; the application is usually performed by hand and sometimes using a syringe. Such a syringe is described in DE-A-28 01 706 or in EP-A1-170 120, for instance. By using a bone cement syringe, markedly better results were achieved in the technique of cementing in view of the cement anchorage in bones than had been achieved with the conventional method in which the bone cement was finger packed.
On the one hand the aim of improving the cementing technique was to improve the anchorage in the bone by impacting the bone cement, on the other, however, one also wanted to solidify the bone cement itself and improve the material properties of the plastic mass. For this reason an attempt was made to compress the bone cement as described, for instance, in EP-A1-170 120. Other authors attempted to remove the air bubbles by evacuating them during the mixing phase as is done with cement used on building sites. However, as the evaporation temperature of the monomer reaches its critical stage at room temperature, monomer bubbles already formed in the bone cement at a pressure of under 400 mbar (40 kPa). This lead to the development of processes (LIDGREN, DRAR and MOELLER, "Polymethylmethacrylate Mixing with Special Reference to Strength", Acta Orthop Scand, 1984, LIDGREN, BODELIND and MOELLER, "Mechanical Properties of Bone Cement with Special Reference to Vacuum Mixing and Chilling", presented at the Swedish Medical Society's Meeting on 30th November 1984) which attempted to prevent the monomer from evaporating by supercooling it and achieved a major reduction in the number of bubbles in the bone cement paste. Due to the reduction of the temperature, however, the process of polymerisation took longer as the chemical reactions took place more slowly. What is more, problems arose from the complicated process of supercooling the monomer and the mixing vessel.